With the alteration of refrigerant gas for refrigerators to hydrogenated fluorocarbons such as Freon R-134a (CH.sub.2 F-CF.sub.3) which is an ozone layer-nondestructive HFC, mineral oils or alkylbenzenes which have heretofore been used as lubricant oil for refrigerators have come not to be used therefor because they have no compatibility with the refrigerant gas. Accordingly, such compounds as polypropylene glycol, polypropylene glycol monoalkyl ether and polypropylene glycol dialkyl ether have come to be used as lubricant oil for refrigerators. However, the above-mentioned compounds are low in compatibility with Freon R-134a, and there was such a problem that among the compounds mentioned above, those having such high viscosity as a kinematic viscosity at 100.degree. C. of more than 15 cSt are particularly low in compatibility with Freon 134a, and hence exhibit low performance as a lubricant oil for refrigerators, for example, a lubricant oil for a rotary automatic air-conditioner.
By the way, polycarbonates are materials which are usable as various lubricant oils or components thereof. However, polycarbonates known heretofore have a volume resistivity of the order of 10.sup.11 to 10.sup.12 .OMEGA..cm, and they were not always sufficient to show good electrical insulation properties when used in the fields of lubricant oils and electrical insulating oils where the electrical insulation properties are required.
Accordingly, there has heretofore been desired not only the advent of highly viscous compounds excellent in lubricating properties as well as electrical insulating properties and also in compatibility with Freon R-134a but also the advent of lubricant oils for refrigerators containing such compounds as mentioned above.
French Patent No. 2,321,477 discloses tricarbonate of trimethylolpropane represented by the following general formula. ##STR1##
In the above general formula, R is branched or non-branched alkyl of 4-12 carbon atoms, cycloalkyl, allyl or aralkyl which may be substituted sometimes with lower alkyl. According to the above-cited French patent, it is alleged that the disclosed tricarbonate can be used as a main component of lubricants stable to heat, and the lubricants containing this tricarbonate may be used particularly in supersonic aircraft.
Of the polycarbonates, particularly those having an alkyl group at the molecular terminals have such an advantage that they are low in hygroscopicity. Japanese Patent L-O-P Publn. No. 3865/1971 discloses a process for preparing such polycarbonates. That is, this Japanese publication teaches that such polycarbonates as mentioned above may be obtained by allowing 1 or 2 mols of a diol to react with 1 or 2 mols of a monoalcohol in a mixture of n mols of the diol and (n-1) mols of diphenyl carbonate. For example, the above-cited publication discloses a process for preparing a polycarbonate having a decyl group at the molecular terminals by allowing n-decanol as the above-mentioned diol to react with polyethylene glycol as the above-mentioned diol while heating under a reduced pressure.
In the above-mentioned process, highly reactive diphenyl carbonate is used preferably as a starting carbonate. The reason why highly reactive diphenyl carbonate is used as the starting carbonate in preference to other carbonates is that this diphenyl carbonate has a high boiling point and so it is easy to preset the reaction conditions therefor.
In contrast thereto, when dimethyl carbonate or diethyl carbonate which is low in reactivity and has a low boiling point is used as the starting carbonate in the above-mentioned process, it is not easy to terminate the molecular terminals of the resulting polycarbonate substantially only with an alkyl group. Furthermore, a catalyst must be used when such a low reactive carbonate as mentioned above is used as the starting carbonate, but it is also not easy to remove the catalyst having a high boiling point from the reaction product.
It is, however, industrially economically advantageous to use, as the starting material, dimethyl carbonate or diethyl carbonate which is available at a low price as compared with diphenyl carbonate.
It has already been known, in general, that a polycarbonate can be obtained by a reaction of a monoalcohol with a carbonate such as dimethyl carbonate.
However, no desired polycarbonate can be obtained in good yield by the prior art process for preparing polycarbonates by allowing a polyol instead of a monoalcohol to react with the carbonate, distilling off the remaining unreacted carbonate by heating after completion of the reaction, and neutralizing the catalyst with an acid, because polymerization of the above-mentioned unreacted carbonate takes place at the stage of distilling off said unreacted carbonate.
As the process for preparing polycarbonates, there has heretofore been known, for example, a process for preparing polycarbonate which comprises reacting a polyol such as diethylene glycol with a carbonate such as dimethyl carbonate by heating in the presence of a basic catalyst, distilling off the unreacted carbonate by heating, and neutralizing the catalyst with an acid such as an organic acid.
In the polycarbonate obtained by the above-mentioned process where the catalyst contained in said polycarbonate is merely neutralized, however, an ionic compound remains therein. Accordingly, the polycarbonates obtained by the above-mentioned process involve such a problem that their characteristics such as electrical resistance properties will deteriorate.
The present invention is intended to solve such problems associated with the prior art as mentioned above, and an object of the invention is to provide polycarbonates excellent in compatibility with an ozone layer-nondestructive hydrogenated fluorocarbon such as Freon R-134a, with hydrogenated chlorofluorocarbon having small ozone destruction power and further with mixtures thereof, particularly highly viscous polycarbonate excellent in compatibility (mutual solubility) with Freon R-134a.
A further object of the invention is to provide polycarbonates excellent not only in compatibility as aforesaid but also in lubricating properties as well as in electrical insulation properties.
Another object of the invention is to provide lubricant oils and electrical insulating oils comprising the above-mentioned polycarbonates, particularly lubricant oils suited for use in refrigerators where an ozone layer-nondestructive Freon is used as a refrigerant, and lubricant oils and electrical insulating oils particularly suited for use in electric refrigerators.
A still further object of the invention is to provide a process for preparing polycarbonates having an alkyl group at the molecular terminals in high yields and in an economical manner by aid of simplified preparative step.
A still another object of the invention is to provide a process for purifying polycarbonates by removing basic substances from the polycarbonates containing the same and thereby obtaining polycarbonates excellent in electrical insulation properties.